Point-to-multipoint digital radio frequency transport

ABSTRACT

A digital radio frequency transport system that performs bi-directional simultaneous digital radio frequency distribution is provided. The transport system includes a digital host unit and at least two digital remote units coupled to the digital host unit. The bi-directional simultaneous digital radio frequency distribution is performed between the digital host unit and the at least two digital remote units.

TECHNICAL FIELD

The present invention is related to high capacity mobile communicationssystems, and more particularly to a point-to-multipoint digitalmicro-cellular communication system.

BACKGROUND INFORMATION

With the widespread use of wireless technologies additional signalcoverage is needed in urban as well as suburban areas. One obstacle toproviding full coverage in these areas is steel frame buildings. Insidethese tall shiny buildings (TSBs), signals transmitted from wirelessbase stations attenuate dramatically and thus significantly impact theability to communicate with wireless telephones located in thebuildings. In some buildings, very low power ceiling mountedtransmitters are mounted in hallways and conference rooms within thebuilding to distribute signals throughout the building. Signals aretypically fed from a single point and then split in order to feed thesignals to different points in the building.

In order to provide coverage a single radio frequency (RF) source needsto simultaneously feeds multiple antenna units, each providing coverageto a different part of a building for example. Simultaneousbi-directional RF distribution often involves splitting signals in theforward path (toward the antennas) and combining signals in the reversepath (from the antennas). Currently this can be performed directly at RFfrequencies using passive splitters and combiners to feed a coaxialcable distribution network. In passive RF distribution systems, signalsplitting in the forward path is significantly limited due to inherentinsertion loss associated with the passive devices. Each split reducesthe level of the signal distributed in the building thereby makingreception, e.g. by cell phones, more difficult. In addition, the highinsertion loss of coaxial cable at RF frequencies severely limits themaximum distance over which RF signals can be distributed. Further, thesystem lacks any means to compensate for variations of insertion loss ineach path.

Another solution to distributing RF signals in TSBs is taking the RFsignal from a booster or base station, down converting it to a lowerfrequency, and distributing it via Cat 5 (LAN) or coaxial cable wiringto remote antenna units. At the remote antenna units, the signal is upconverted and transmitted. While down-conversion reduces insertion loss,the signals are still susceptible to noise and limited dynamic range.Also, each path in the distribution network requires individual gainadjustment to compensate for the insertion loss in that path.

In another approach, fiber optic cables are used to distribute signalsto antennas inside of a building. In this approach, RF signals arereceived from a bi-directional amplifier or base station. The RF signalsdirectly modulate an optical signal, which is transported throughout thebuilding as analog modulated light signals over fiber optic cable.Unfortunately, conventional systems using analog optical modulationtransmission over optical fibers require highly sophisticated linearlasers to achieve adequate performance. Also, analog optical systems arelimited in the distance signals can be transmitted in the building.Typically, this limitation is made worse due to the use of multimodefiber that is conventionally available in buildings. Multimode fiber iswider than single mode fiber and supports a number of differentreflection modes so that signals tend to exhibit dispersion at theterminating end of the fiber. In addition, analog installation typicallyincludes significant balancing when setting up the system. Further, RFlevels in the system need to be balanced with the optical levels. Ifthere is optical attenuation, the RF levels need to be readjusted. Inaddition, if the connectors are not well cleaned or properly secured,the RF levels can change.

Digitization of the RF spectrum prior to transport solves many of theseproblems. The level and dynamic range of digitally transported RFremains unaffected over a wide range of path loss. This allows for muchgreater distances to be covered, and eliminates the path losscompensation problem. However, this has been strictly a point-to-pointarchitecture. One drawback with digitally transported RF in apoint-to-point architecture is the equipment and cost requirement. Ahost RF to digital interface device is needed for each remote antennaunit. In particular, for use within a building or building complex thenumber of RF to digital interface devices and the fiber to connect thesedevices is burdensome. For example, in a building having 20 floors, therequirement may include 20 host RF to digital interface devices for 20remote antenna units, 1 per floor. In some applications more than oneremote antenna unit per floor may be required. As a result, there is aneed in the art for improved techniques for distributing RF signals inTSBs, which would incorporate the benefits of digital RF transport intoa point-to-multipoint architecture.

SUMMARY OF THE INVENTION

The above-mentioned problems with distributing RF signals within abuilding and other problems are addressed by the present invention andwill be understood by reading and studying the following specification.

In one embodiment, a digital radio frequency transport system isprovided. The transport system includes a digital host unit and at leasttwo digital remote units coupled to the digital host unit. The digitalhost unit includes shared circuitry that performs bi-directionalsimultaneous digital radio frequency distribution between the digitalhost unit and the at least two digital remote units.

In another embodiment, a digital radio frequency transport system isprovided. The transport system includes a digital host unit and at leastone digital expansion unit coupled to the digital host unit. Thetransport system further includes at least two digital remote units,each coupled to one of the digital host unit and the digital expansionunit. The digital host unit includes shared circuitry that performsbi-directional simultaneous digital radio frequency distribution betweenthe digital host unit and the at least two digital remote units.

In an alternate embodiment, a method of performing point-to-multipointradio frequency transport is provided. The method includes receivingradio frequency signals at a digital host unit and converting the radiofrequency signals to a digitized radio frequency spectrum. The methodalso includes optically transmitting the digitized radio frequencyspectrum to a plurality of digital remote units. The method furtherincludes receiving the digitized radio frequency spectrum at theplurality of digital remote units, converting the digitized radiofrequency spectrum to analog radio frequency signals and transmittingthe analog radio frequency signals via a main radio frequency antenna ateach of the plurality of digital remote units.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of one embodiment of a point-to-multipointcommunication system according to the teachings of the presentinvention.

FIG. 2 is a block diagram of one embodiment of a communication systemaccording to the teachings of the present invention.

FIG. 3 is a block diagram of another embodiment of a communicationsystem according to the teachings of the present invention.

FIG. 4 is a block diagram of one embodiment of a digital host unitaccording to the teachings of the present invention.

FIG. 5 is a block diagram of one embodiment of a digital remote unitaccording to the teachings of the present invention.

FIG. 6 is a block diagram of one embodiment of a digital expansion unitaccording to the teachings of the present invention.

FIG. 7 is a block diagram of one embodiment of a microcell base stationaccording to the teachings of the present invention.

FIG. 8 is an illustration of one embodiment of an overflow algorithm fora channel summer according to the teachings of the present invention.

DETAILED DESCRIPTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which is shown by way of illustration specific embodiments inwhich the invention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

FIG. 1 is an illustration of one exemplary embodiment of apoint-to-multipoint digital transport system shown generally at 100 andconstructed according to the teachings of the present invention. Thepoint-to-multipoint digital transport system 100 is shown distributedwithin a complex of tall shiny buildings (TSBs) 2. Although system 100is shown in a complex of TSBs 2, it is understood that system 100 is notlimited to this embodiment. Rather, system 100 in other embodiments isused to distribute signals in a single building, or other appropriatestructure or indoor or outdoor location that exhibits high attenuationto RF signals. Advantageously, system 100 uses digital summing ofdigitized RF signals from multiple antennas to improve signal coveragein structures, such as TSBs.

Point-to-multipoint digital transport of RF signals is accomplishedthrough a network of remote antenna units or digital remote units 40 and40′ and a digital host unit 20, which interfaces with a wireless network5 which is coupled to the public switched telephone network (PSTN), or amobile telecommunications switching office (MTSO) or other switchingoffice/network. System 100 operates by transporting RF signals digitallyover fiber optic cables. Signals received at DHU 20 are distributed tomultiple DRUs 40 and 40′ to provide coverage throughout a buildingcomplex. In addition, signals received at each of the DRUs 40 and 40′are summed together at the DHU 20 for interface to a wireless network.

In one embodiment, digital expansion unit DEU 30 is situated between theDHU 20 and one or more DRUs. In the forward path, DEU 30 expands thecoverage area by splitting signals received from DHU 20 to a pluralityof DRUs 40′. In the reverse path, DEU 30 receives signals from aplurality of DRUs 40′, digitally sums the signals together andtransports them to a DHU 20 or another DEU such as 30. This systemallows for successive branching of signals using DEUs 30 and expandedcoverage to multiple DRUs 40 and 40′. This system provides an efficientway of providing signal coverage for wireless communication withoutadded attenuation loss and distance constraint found with analogsystems. By using DEUs 30, antennas can be placed further from DHU 20without adversely affecting signal strength since shorter fiber opticcables can be used.

Digital transport system 100 includes a wireless interface device (WID)10 that provides an interface to a wireless network. In one embodiment,the WID 10 includes either conventional transmitters and receivers orall digital transmitter and receiver equipment, and interface circuitryto a mobile telecommunications switching office (MTSO). In oneembodiment, the wireless interface device 10 is coupled to an MTSO via aT1 line and receives and transmits signals between the MTSO and the DHU20. In another embodiment, the wireless interface device 10 is coupledto the public switched telephone network (PSTN). In one embodiment, WID10 comprises a base station and connects directly to DHU 20 via coaxialcables. In another embodiment, WID 10 comprises a base station andwirelessly connects to DHU 20 via a bi-directional amplifier that isconnected to an antenna. In one embodiment, the antenna is an outdoorantenna.

WID 10 communicates signals between wireless units and the wirelessnetwork via digital remote units DRUs 40 and 40′. WID 10 is coupled toDHU 20. The DHU 20 is coupled to at least one digital expansion unit DEU30 and a plurality of DRUs 40. In addition, DEU 30 is coupled to aplurality of DRUs 40′. The DHU 20 receives RF signals from WID 10 andconverts the RF signals to digital RF signals. DHU 20 further opticallytransmits the digital RF signals to multiple DRUs 40 either directly orvia one or more DEUs 30.

Each DRU 40 and 40′ is connected through a fiber optic cable (oroptionally another high bandwidth carrier) to transport digital RFsignals to one of DHU 20 or DEU 30. In one embodiment, the fiber opticcable comprises multimode fiber pairs coupled between the DRUs 40 andthe DHU 20, between the DRUs 40 and 40′ and the DEUs 30 and between theDEUs 30 and the DHU 20. In one embodiment, the DEU 30 is coupled to theDHU 20 via single mode fiber and the DEU 30 is coupled to the DRUs 40′via multimode fiber pairs. Although, transport system 100 has beendescribed with fiber optic cable other carriers may be used, e.g.,coaxial cable.

In another embodiment, the DHU 20 is coupled to the DRUs 40 by a directcurrent power cable in order to provide power to each DRU 40. In oneembodiment, the direct current power cable delivers 48 VDC to each DRU40 connected to the DHU 20. In another embodiment, the DEU 30 is coupledto DRUs 40′ by a direct current power cable to provide power to each DRU40′. In one embodiment, the direct current power cable delivers 48 VDCto each DRU 40′ connected to the DEU 30. In an alternate embodiment,DRUs 40 and 40′ are connected directly to a power supply. In oneembodiment, the power supply provides DC power to the DRUs 40 and 40′.In an alternate embodiment, the power supply provides AC power to theDRUs 40 and 40′. In one embodiment, DRUs 40 and 40′ each include anAC/DC power converter.

Both DHU 20 and DEU 30 split signals in the forward path and sum signalsin the reverse path. In order to accurately sum the digital signalstogether at DHU 20 or DEU 30 the data needs to come in to the DHU 20 orDEU 30 at exactly the same rate. As a result all of the DRUs 40 and 40′need to be synchronized so that their digital sample rates are alllocked together. Synchronizing the signals in time is accomplished bylocking everything to the bit rate over the fiber. In one embodiment,the DHU 20 sends out a digital bit stream and the optical receiver atthe DEU 30 or DRU 40 detects that bit stream and locks its clock to thatbit stream. In one embodiment, this is being accomplished with amultiplexer chip set and local oscillators, as will be described below.Splitting and combining the signals in a digital state avoids thecombining and splitting losses experienced with an analog system. Inaddition, transporting the digital signals over multimode fiber resultsin a low cost transport system that is not subject to much degradation.

The down-conversion and up-conversion of RF signals are implemented bymixing the signal with a local oscillator (LO) at both the DRUs and theDHU. In order for the original frequency of the RF signal to berestored, the signal must be up-converted with an LO that has exactlythe same frequency as the LO that was used for down conversion. Anydifference in LO frequencies will translate to an equivalent end-to-endfrequency offset. In the embodiments described, the down conversion andup conversion LOs are at locations remote from one another. Therefore,in one preferred embodiment, frequency coherence between the local andremote LO's is established as follows: at the DHU end, there is a 142MHz reference oscillator which establishes the bit rate of 1.42 GHz overthe fiber. This reference oscillator also generates a 17.75 MHzreference clock which serves as a reference to which LO's at the DHU arelocked.

At each of the DRUs, there is another 17.75 MHz clock, which isrecovered from the optical bit stream with the help of the clock and bitrecovery circuits. Because this clock is recovered from the bit streamgenerated at the host, it is frequency coherent with the referenceoscillator at the host. A reference 17.75 MHz clock is then generated toserve as a reference for the remote local oscillators. Because theremote recovered bit clock is frequency coherent with the host masterclock, the host and remote reference clocks, and any LO's locked tothem, are also frequency coherent, thus ensuring that DHU and DRU LO'sare locked in frequency. It is understood that in other embodiments thebit rate over the fiber may vary and the frequency of the clocks willalso vary.

FIG. 2 is a block diagram of one embodiment of a communication system,shown generally at 200 and constructed according to the teachings of thepresent invention. In this embodiment, a digital host unit (DHU) 220 iscoupled to a bi-directional amplifier (BDA) 211. The BDA 211 receivescommunication signals from a wireless interface device (WID) andtransports the communication signals as RF signals to the DHU 220 andreceives RF signals from DHU 220 and transmits the RF signals to theWID. The DHU 220 receives RF signals from the BDA 211 and digitizes theRF signals and optically transmits the digital RF signals to multipleDRUs via transmission lines 214-1 to 214-N. DHU 220 also receivesdigitized RF signals over transmission lines 216-1 to 216-N from aplurality of DRUs either directly or indirectly via DEUs, reconstructsthe corresponding analog RF signals, and applies them to BDA 211. In oneembodiment, DHU 220 receives signals directly from a plurality N ofDRUs. The signals are digitally summed and then converted to analogsignals and transmitted to BDA 211. In another embodiment, DHU 220receives signals from one or more DEUs and one or more DRUs directly.Again, the signals are all digitally summed and then converted to analogsignals and transmitted to BDA 211. The signals received viatransmission lines 216-1 to 216-N may be received directly from a DRU orsignals that are received by a DEU and summed together and thentransported via 216-1 to 216-N to DHU 220 for additional summing andconversion for transport to BDA 211. DEUs provide a way to expand thecoverage area and digitally sum signals received from DRUs or other DEUsfor transmission in the reverse path to other DEUs or DHU 220. In oneembodiment, transmission lines 214-1 to 214-N and 216-1 to 216-Ncomprise multimode fiber pairs. In an alternate embodiment, each fiberpair is replaced by a single fiber, carrying bi-directional opticalsignals through the use of wavelength division multiplexing (WDM). In analternate embodiment, transmission lines 214-1 to 214-N and 216-1 to216-N comprise single mode fibers. In one embodiment, N is equal to six.In an alternate embodiment, the number of transmission lines in theforward path direction 214-1 to 214-N is not equal to the number oftransmission lines in the reverse path direction 216-1 to 216-N.

FIG. 3 is a block diagram of an alternate embodiment of a communicationsystem shown generally at 300 and constructed according to the teachingsof the present invention. Communication system 300 includes a basestation 310 coupled to a DHU 320. Base station 310 includes conventionaltransmitters and receivers 323 and 328, respectively, and conventionalradio controller or interface circuitry 322 to an MTSO or telephoneswitched network. DHU 320 is coupled to base station 310. DHU 320 isalso coupled to transmission lines 314-1 to 314-M, which transmit in theforward path direction and transmission lines 316-1 to 316-M, whichtransmit in the reverse path direction.

DHU 320 essentially converts the RF spectrum to digital in the forwardpath and from digital to analog in the reverse path. In the forwardpath, DHU 320 receives the combined RF signal from transmitters 323,digitizes the combined signal and transmits it in digital format overfibers 314-1 to 314-M, which are connected directly to a plurality ofDRUs or indirectly to one or more DRUs via one or more DEUs.

In one embodiment, DHU 320 receives signals directly from a plurality Mof DRUs. The signals are digitally summed and then converted to analogsignals and transmitted to base station 310. In another embodiment, DHU320 receives signals from one or more DEUs and one or more DRUsdirectly. Again, the signals are all digitally summed and then convertedto analog signals and transmitted to base station 310. The signalsreceived via transmission lines 316-1 to 316-M may be received directlyfrom a DRU or signals that are received by a DEU and summed together andthen transported via 316-1 to 316-M to DHU 320 for additional summingand conversion for transport to base station 210. DEUs provide a way toexpand the coverage area by splitting signals in the forward path anddigitally summing signals received from DRUs or other DEUs in thereverse path for transmission upstream to other DEUs or a DHU. In thereverse path, DHU 320 also receives digitized RF signals over fibers316-1 to 316-M from a plurality of DRUs, either directly or indirectlyvia DEUs, reconstructs the corresponding analog RF signal, and appliesit to receivers 328.

In one embodiment, transmission lines 314-1 to 314-M and 316-1 to 316-Mcomprise multimode fiber pairs. In an alternate embodiment, each fiberpair is replaced by a single fiber, carrying bi-directional opticalsignals through the use of wavelength division multiplexing (WDM). In analternate embodiment, transmission lines 314-1 to 314-M and 316-1 to316-M comprise single mode fibers. In one embodiment, M is equal to six.In an alternate embodiment, the number of transmission lines in theforward path direction 314-1 to 314-M is not equal to the number oftransmission lines in the reverse path direction 316-1 to 316-M.

Referring now to FIG. 4, there is shown one embodiment of a DHU 420constructed according to the teachings of the present invention. DHU 420includes an RF to digital converter 491 receiving the combined RFsignals from a wireless interface device such as a base station, BDA orthe like. RF to digital converter 491 provides a digitized trafficstream that is transmitted to multiplexer 466. Multiplexer 466 convertsthe parallel output of the A/D converter into a framed serial bitstream. At the output of the multiplexer is a 1 to P fan out buffer 407,which splits the digital signal P ways. There are P optical transmitters431-1 to 431-P one feeding each of the P optical transmission lines414-1 to 414-P. The digitized signals are applied to fibers 414-1 to414-P for transmission to corresponding DRUs either directly or viaDEUs. In one embodiment, P is equal to 6.

In one embodiment, DHU 420 includes an amplifier 450 that receives thecombined RF signal from a wireless interface device such as a basestation or BDA. The combined RF signal is amplified and then mixed bymixer 452 with a signal received from local oscillator 468. Localoscillator 468 is coupled to reference oscillator 415. In one embodimentthe local oscillator is coupled to a frequency divider circuit 470,which is in turn coupled to reference oscillator 415. The localoscillator is locked to the reference oscillator 415 as a master clockso that the down conversion of the RF signals is the same as the upconversion. The result is end to end, from DHU to DRU, or DHU to one ormore DEUs to DRU, no frequency shift in the signals received andtransmitted. The local oscillator 463 is also coupled to a synthesizercircuit 476.

The output signal of mixer 452 is provided to amplifier 454 amplifiedand then filtered via intermediate frequency (IF) filter 456. Theresultant signal is the combined RF signal converted down to an IFsignal. The IF signal is mixed with another signal originating from thereference oscillator 415 via mixer 460. The output of mixer 460 issummed together at 462 with a signal produced by field programmable gatearray (FPGA) 467. The output is then converted from an analog signal toa digital signal via analog/digital (A/D) converter 464 once convertedthe digital RF signal is applied to multiplexer 466. In one embodiment,the A/D converter 464 is a 14-bit converter handling a 14-bit signal. Inother embodiments, the AID converter 464 may be of any size toaccommodate an appropriate signal. In one embodiment, the input signalfrom FPGA 467 is a dither signal from dither circuit 462 that addslimited out of band noise to improve the dynamic range of the RF signal.

In one embodiment, DHU 420 includes an alternating current to digitalcurrent power distribution circuit 6 that provides direct current powerto each of the DRUs coupled to DHU 420.

DHU 420 further includes a plurality of digital optical receivers 418-1to 418-P in the reverse path. Receivers 418-1 to 418-P each output anelectronic digital signal, which is applied to clock and bit recoverycircuits 445-I to 445-P, respectively, for clock and bit recovery of theelectronic signals. The signals are then applied to demultiplexers 441-1to 441-P, respectively, which extract the digitized signals generated atthe DRUs, as will be explained in detail below. Demultiplexers 441-1 to441-P further extract alarm (monitoring) and voice information framedwith the digitized signals. The digitized signals output at eachdemultiplexer 441-1 to 441-P are then applied to FPGA 467 where thesignals are summed together and then applied to digital to RF converter495. Converter 495 operates on the sum of the digitized signalsextracted by demultiplexers 441-1 to 441-P, reconstructing basebandreplicas of the RF signals received at all the digital remote units. Thebaseband replicas are then up-converted to their original radiofrequency by mixing with a local oscillator 482 and filtering to removeimage frequencies. Local oscillator 482 is coupled to synthesizer 476and reference oscillator as discussed with respect to local oscillator468 above.

In one embodiment, digital to RF converter 495 includes digital toanalog (D/A) converter 484 coupled to an output of FPGA 467 thedigitized RF signals are converted to analog RF signals and then mixedwith a signal from reference oscillator 415 by mixer 492. The signal isthen filtered by IF filter 490 and amplified by amplifier 488. Theresultant signal is then mixed with a signal from local oscillator 482and then applied to RF filter 484, amplifier 480 and RF filter 478 fortransmission by a wireless interface device such as a BDA or basestation.

In one embodiment, FPGA 467 includes an alarm/control circuit 474, whichextracts overhead bits from DRUs to monitor error and alarm information.In one embodiment, the FPGA 467 includes a summer 498, whichmathematically sums together the digital RF signals received from fibers416-1 to 416-P. In another embodiment FPGA includes an overflowalgorithm circuit 486 coupled to the output of summer 486. The algorithmcircuit 496 allows the summed digital RF signals to saturate and keepthe summed signal within a defined number of bits. In one embodiment,the algorithm circuit includes a limiter. In one embodiment, the RFsignals are 14-bit signals and when summed and limited by summer 498 andoverflow algorithm 496 result in a 14-bit output signal.

For example, in one embodiment each of the digital RF signals receivedfrom fibers 416-1 to 416-P, where P is equal to 6, comprise 14 bitinputs. All of those 6 different 14 bit inputs then go into summer 498.In order to allow for overflow, at least 17 bits of resolution is neededin the summer 498 to handle a worst-case scenario when all 6 of the 14bit inputs are at full scale at the same time. In this embodiment, a17-bit wide summer 498 is employed to handle that dynamic range. Comingout of summer 498 is needed a 14-bit signal going in the reverse path.In one embodiment, an algorithm circuit 496 for managing the overflow isimplemented. In one embodiment, the summer and 498 and overflowalgorithm 496 are included in FPGA 467. In one embodiment, overflowalgorithm 496 acts like a limiter and allows the sum to saturate andkeeps the summed signal within 14 bits. In an alternate embodiment,overflow algorithm circuit 496 controls the gain and scales the signaldynamically to handle overflow conditions.

FIG. 8 illustrates one embodiment of an algorithm 863 for a channelsummer 865 in order to limit the sum of input signals 0 to 5 to 14 bits.In this embodiment, input signals 0 to 5 comprise 6 signals that aresummed together by summer 865. The sum of input signals 0 to 5 isreduced to a signal having 14 bits or less by the algorithm 863. It isunderstood that the algorithm 865 is by example and is not meant torestrict the type of algorithm used to limit the sum of signals 0 to 5to 14 bits or less.

For example, when the sum of the 6 input signals 0 to 5 is greater thanor equal to 13FFBh then the sum is divided by 6 for a signal that is 14bits or less. When the sum of the 6 input signals 0 to 5 is greater than13FFBh but less than or equal to FFFCh then the sum is divided by 5 fora signal that is 14 bits or less. When the sum of the 6 input signals 0to 5 is greater than FFFCh but less than BFFDh then the sum is dividedby 4 for a signal that is 14 bits or less. When the sum of the 6 inputsignals 0 to 5 is greater BFFDh but less than 7FFEh then the sum isdivided by 3 for a signal that is 14 bits or less. Finally, when the sumof the 6 input signals 0 to 5 is greater than 7FFEh but less than orequal to 3FFFh then the sum is divided by 2 for a signal that is 14 bitsor less.

FIG. 5 is a block diagram of one embodiment of a digital remote unit(DRU) 540 constructed according to the teachings of the presentinvention. A digital optical receiver 501 receives the optical digitaldata stream transmitted from a DHU directly or via a DEU. Receiver 501converts the optical data stream to a corresponding series of electricalpulses. The electrical pulses are applied to clock and bit recoverycircuit 503. The series of electrical pulses are then applied todemultiplexer 505. Demultiplexer 505 extracts the digitized trafficsignals and converts the signals from serial to parallel. The outputparallel signal is then applied to digital to RF converter 595 forconversion to RF and transmission to duplexer 547. RF converter 595 isconnected to the main antenna 599 through a duplexer 547. Accordingly,radio frequency signals originating from a wireless interface device aretransmitted from main antenna 547.

In one embodiment, digital to RF converter 595 includes adigital-to-analog (D/A) converter 509, which reconstructs the analog RFsignal and applies it to IF 504 and amplifier 506. The analog signal ismixed with an output signal of reference oscillator 515 by mixer 502.The output of amplifier 506 is mixed with a signal from local oscillator519 that locks the RF signal with the return digital signal viareference oscillator 515 that is coupled to local oscillator 519. In oneembodiment, the reference oscillator is coupled to frequency divider 517that in turn is coupled to local oscillators 519 and 529. The localoscillators 519 and 529 are also coupled to synthesizer 521 that iscoupled to programmable logic device 525.

RF signals received at main antenna 599 are passed through duplexer 547to RF to digital converter 593. The RF signals are converted to digitalsignals and then applied to multiplexer 536 converted fromparallel-to-serial and optically transmitted via optical transmitter 532to a DEU or DHU.

In one embodiment, RF to digital converter 593 includes a firstamplifier 543 that receives RF signals from duplexer 547, amplifies thesignals and transmits them to digital attenuator 539. In one embodiment,amplifier 543 is a low noise amplifier. Digital attenuator 539 receivesthe amplified signals and digitally attenuates the signal to control thelevels in case of an overload situation. RF to digital converter 593further includes a second amplifier 537, which receives the attenuatedsignals, amplifies the signals and applies the amplified signals tomixer 535. Mixer 535 mixes the amplified signals with a signal receivedfrom local oscillator 529. The resultant signals are applied to a thirdamplifier 533 an IF filter 548 and a fourth amplifier 546 in series todown convert to an IF signal. The IF signal is then mixed with a signalfrom reference oscillator 515 and the mixed signal is summed with asignal from dither circuit 527. The resultant signal is applied toanalog-to-digital converter 538 and converted to a digital signal. Theoutput digital signal is then applied to a multiplexer 536. In oneembodiment, the multiplexer 536 multiplexes the signal together with acouple of extra bits to do framing and control information. In oneembodiment, multiplexer 536, clock and bit recovery circuit anddemultiplexer 505 comprise a multiplexer chip set.

Programmable logic circuit 525 programs synthesizer 521 for thereference oscillator and for the up and down conversion of localoscillators 519 and 529. The programmable logic circuit 525 looks forerror conditions, for out of lock conditions on the oscillators andreports error modes and looks for overflow condition in the A/Dconverter 538. If an overflow condition occurs the programmable logiccircuit 525 indicates that you are saturating and adds some extraattenuation at digital attenuator 539 in order to reduce the RF signallevels coming in from RF antenna 599 and protect the system fromoverload.

In one embodiment, DRU 540 includes an internal direct current powerdistribution system 5. In one embodiment, the distribution systemreceives 48 VDC and internally distributes 3 outputs of +3.8V, +5.5V and+8V.

FIG. 6 is a block diagram of one embodiment of a digital expansion unit(DEU) 630 constructed according to the teachings of the presentinvention. DEU 630 is designed to receive optical signals and transmitoptical signals. An optical receiver 651 receives digitized RF signalsand transmits them to clock and bit recovery circuit 653 that performsclock and bit recovery to lock the local clock and clean up the signal.The signals are then split into X RF digital signals by 1 to X fan outbuffer 607. The signals are then transmitted via optical transmitters655-1 to 655-X to X receiving units such as DEUs or DRUs. The Xreceiving units may be any combination of DEUs or DRUs. In oneembodiment, X is equal to six.

DEU 630 also includes optical receivers 669-1 to 669-X, which receivedigitized RF signals directly from DRUs or indirectly via DEUs. Inoperation the signals are received, applied to clock and bit recoverycircuits 673-1 to 673-X respectively to lock the local clock and cleanup the signals and then applied to demultiplexers 671-1 to 671-X.Demultiplexers 671-1 to 671-X each extract the digitized traffic andapply the samples to field programmable gate array 661. The signals aresummed together digitally and transmitted to multiplexer 657, whichmulitplexes the signal together with a couple of extra bits to doframing and control information. In addition, the multiplexer 657converts the signals parallel to serial. The signals are then applied tooptical transmitter 659 for further transmission. In one embodiment, thesignals are directly transmitted to a DHU or indirectly via one or moreadditional DEUs.

In one embodiment, the FPGA 661 includes summer 665, whichmathematically sums together the digital RF signals received fromdemultiplexers 671-1 to 671-X. In another embodiment, FPGA 661 includesan overflow algorithm circuit 663 coupled to the output of summer 665.The algorithm circuit 663 allows the summed digital RF signals tosaturate and keep the summed signal within a defined number of bits. Inone embodiment, the algorithm circuit includes a limiter. In oneembodiment, the RF signals are 14-bit signals and when summed andlimited by summer 665 and overflow algorithm 663 result in a 14-bitoutput signal.

In one embodiment, DEU 630 includes an alternating current to digitalcurrent power distribution circuit 7 that provides direct current powerto each of the DRUs coupled to DEU 630.

In an alternate embodiment, the digital host unit (DHU) and wirelessinterface device (WID) are located some distance from the building beingserved. The DHU in the building is replaced by a DEU, and the linkbetween that DEU and the remotely located DHU is via single mode fiber.FIG. 7 is a block diagram of this embodiment. A microcell base stationshown generally at 700 includes conventional transmitters and receivers723 and 728, respectively, and conventional radio controller orinterface circuitry 722. In the forward path, a DHU 767 receives thecombined RF signal from transmitters 723, digitizes the combined signaland transmits it in digital format over single mode fiber to a DEU. Inthe reverse path, DHU 767 receives digitized RF signal from a DEU,reconstructs the corresponding analog RF signal, and applies it toreceivers 728.

In another alternate embodiment, the wireless interface device (WID) isa software defined base station, and the interface between the DHU andWID takes place digitally, eliminating the need for the RF to digitalconversion circuitry in the DHU.

Conclusion

A digital radio frequency transport system has been described. Thetransport system includes a digital host unit and at least two digitalremote units coupled to the digital host unit. The digital host unitincludes shared circuitry that performs bi-directional simultaneousdigital radio frequency distribution between the digital host unit andthe at least two digital remote units.

In addition, a digital radio frequency transport system has beendescribed. The transport system includes a digital host unit and atleast one digital expansion unit coupled to the digital host unit. Thetransport system further includes at least two digital remote units,each coupled to one of the digital host unit and the digital expansionunit. The digital host unit includes shared circuitry that performsbi-directional simultaneous digital radio frequency distribution betweenthe digital host unit and that at least two digital remote units.

Further, a method of performing point-to-multipoint radio frequencytransport has been described. The method includes receiving analog radiofrequency signals at a digital host unit and converting the analog radiofrequency signals to digitized radio frequency signals. The method alsoincludes splitting the digitized radio frequency signals into aplurality of a digital radio frequency signals and opticallytransmitting the digital radio frequency signals to a plurality ofdigital remote units. The method further includes receiving the digitalradio frequency signals at a plurality of digital remote units,converting the digital radio frequency signals to analog radio frequencysignals and transmitting the signals via a main radio frequency antennaat each of the plurality of digital remote units.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiments shown. For example, adigital remote unit is not limited to the receipt and summing andsplitting and transmitting of digitized radio frequency signals. Inother embodiments, the digital host unit is capable of receiving andsumming analog radio frequency signals in addition to or instead ofdigitized radio frequency signals. As well, the digital host unit iscapable of splitting and transmitting analog radio frequency signals inaddition to or instead of digitized radio frequency signals. Thisapplication is intended to cover any adaptations or variations of thepresent invention. Therefore, it is intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. A digital radio frequency transport system,comprising: a digital host unit; and at least two digital remote unitscoupled to the digital host unit, wherein the digital host unit includesshared circuitry that performs bi-directional simultaneous digital radiofrequency distribution between the digital host unit and the at leasttwo digital remote units; wherein the digital host unit includes: atleast two optical receivers each coupled to one of the at least twodigital remote units; at least two clock and bit recovery circuits eachcoupled to an output of one of the at least two optical receivers; andat least two demultiplexers each coupled to an output of one of the atleast two clock and bit recovery circuits.
 2. The system of claim 1,further comprising a wireless interface device coupled to the digitalhost unit.
 3. The system of claim 2, wherein the wireless interfacedevice comprises a base station that couples directly to the digitalhost unit via coaxial cables.
 4. The system of claim 2, wherein thewireless interface device comprises a base station that wirelesslyconnects to the digital host unit via bi-directional amplifier that iscoupled to an antenna.
 5. The system of claim 1, wherein the at leasttwo digital remote units each include a main radio frequency antennawhich transmits and receives radio frequency signals.
 6. The system ofclaim 1, wherein the digital host unit includes a radio frequency todigital converter that converts a main radio frequency signal to adigitized radio frequency signal.
 7. The system of claim 6, wherein thedigital host unit further includes a multiplexer which splits thedigitized radio frequency signal into at least two digital signals foroptical transmission to the at least two digital remote units.
 8. Thesystem of claim 7, wherein the digital host unit further comprises analarm/control interface circuit coupled to the multiplexer.
 9. Thesystem of claim 1, wherein the digital host unit includes localoscillators coupled to a reference oscillator for synchronization of theradio frequency signal in the forward direction and in the reversedirection.
 10. The system of claim 1, wherein the digital host unit iscoupled to each of the at least two digital remote units by a multimodefiber pair.
 11. The system of claim 1, wherein the digital host unitfurther includes a field programmable gate array coupled to an output ofeach of the at least two demultiplexers, wherein the field programmablegate array receives a digital radio frequency signal from each of the atleast two demultiplexers.
 12. The system of claim 11, wherein the fieldprogrammable gate array includes a summer that sums together the digitalradio frequency signals from each of the at least two demultiplexers.13. The system of claim 12, wherein the field programmable gate arrayfurther includes an overflow algorithm circuit, which allows the summeddigital radio frequency signals to saturate and keeps the summed signalwithin a defined number of bits.
 14. The system of claim 13, wherein theoverflow algorithm circuit comprises a limiter.
 15. The system of claim12, wherein the digital host unit further includes a digital to radiofrequency converter coupled to an output of the field programmable gatearray, wherein the radio frequency converter receives the summed digitalradio frequency signal and converts it to an analog radio frequencysignal.
 16. The system of claim 15, wherein the digital to radiofrequency converter comprises a digital to analog converter.
 17. Thesystem of claim 1, wherein the at least two digital remote units eachcomprise: a main radio frequency antenna; a duplexer coupled to the mainradio frequency antenna; a radio frequency to digital converter coupledto an output of the duplexer; a multiplexer coupled to an output of theradio frequency to digital converter; and an optical transmitter coupledto an output of the multiplexer.
 18. A digital radio frequency transportsystem, comprising: a digital host unit, and at least two digital remoteunits coupled to the digital host unit, wherein the digital host unitincludes shared circuitry that performs bi-directional simultaneousdigital radio frequency distribution between the digital host unit andthe at least two digital remote units; wherein the at least two digitalremote units each comprise: an optical receiver; a clock and bitrecovery circuit coupled to an output of the optical receiver; ademultiplexer coupled to an output of the clock and bit recoverycircuit; a digital to radio frequency converter coupled to the output ofthe demultiplexer; a duplexer coupled to an output of the digital toradio frequency converter; and a main radio frequency antenna coupled toan output of the duplexer.
 19. The system of claim 18, wherein the atleast two digital remote units each further comprise: a duplexer coupledto the main radio frequency antenna; a radio frequency to digitalconverter coupled to an output of the duplexer; a multiplexer coupled toan output of the radio frequency to digital converter; and an opticaltransmitter coupled to an output of the multiplexer.
 20. The system ofclaim 19, wherein the digital to radio frequency converter includes adigital to analog converter.
 21. The system of claim 19, wherein theradio frequency converter includes an analog to digital converter. 22.The system of claim 19, wherein the at least two digital remote unitseach further comprise a programmable logic circuit coupled to an outputof the radio frequency to digital converter and a reference oscillatorcoupled between the programmable logic circuit and the clock and bitrecovery circuit, wherein the programmable logic circuit monitors thesystem for error conditions and reports error modes.
 23. The system ofclaim 22, wherein the at least two digital remote units each furthercomprise a synthesizer circuit coupled to an output of the programmablelogic device and a first and a second local oscillator coupled to afirst and a second output, respectively, of the synthesizer.
 24. Adigital radio frequency transport system, comprising: a digital hostunit; at least two digital remote units coupled to the digital hostunit, wherein the digital host unit includes shared circuitry thatperforms bi-directional simultaneous digital radio frequencydistribution between the digital host unit and the at least two digitalremote units; and a digital expansion unit coupled to the digital hostunit, wherein the digital expansion unit contains circuitry thatperforms bi-directional simultaneous digital radio frequencydistribution between the digital host unit and the digital remote unit;wherein the digital expansion unit comprises: an optical receiver; aclock and bit recovery circuit coupled to an output of the opticalreceiver; and a plurality of optical transmitters coupled to an outputof the clock and bit recovery circuit, wherein the optical transmittersare each coupleable to a digital remote unit or a digital expansionunit.
 25. The system of claim 24, wherein the digital expansion unit iscoupled to the digital host unit by a multimode fiber pair.
 26. Thesystem of claim 24, wherein the digital expansion unit is coupled to thedigital host unit by single mode fiber.
 27. A digital radio frequencytransport system, comprising: a digital host unit; at least two digitalremote units coupled to the digital host unit, wherein the digital hostunit includes shared circuitry that performs bi-directional simultaneousdigital radio frequency distribution between the digital host unit andthe at least two digital remote units; and a digital expansion unitcoupled to the digital host unit, wherein the digital expansion unitcontains circuitry that performs bi-directional simultaneous digitalradio frequency distribution between the digital host unit and thedigital remote unit; wherein the digital expansion unit comprises: aplurality of optical receivers; a plurality of clock and bit recoverycircuits each coupled to an output of one of the plurality of opticalreceivers; a plurality of demultiplexers each coupled to an output ofone of the clock and bit recovery circuits; a summer coupled to anoutput of each of the plurality of demultiplexers; a multiplexer coupledto an output of the field programmable gate array; and an opticaltransmitter coupled to an output of the multiplexer.
 28. The system ofclaim 27, wherein the summer comprises a field programmable gate arraythat sums together digital radio frequency signals from the plurality ofdemultiplexers.
 29. The system of claim 28, wherein the fieldprogrammable gate array includes an overflow algorithm circuit, whichallows the summed digital radio frequency signals to saturate and keepsthe summed signal within a defined number of bits.
 30. The system ofclaim 29, wherein the overflow algorithm circuit comprises a limiter.31. A digital radio frequency transport system, comprising: a digitalhost unit; at least one digital expansion unit coupled to the digitalhost unit; and at least two digital remote units, each coupled to one ofthe digital host unit and the digital expansion unit, wherein thedigital host unit includes shared circuitry that performs bi-directionalsimultaneous digital radio frequency distribution between the digitalhost unit and the at least two digital remote units; wherein the atleast one digital expansion unit comprises: an optical receiver; a clockand bit recovery circuit coupled to an output of the optical receiver;and a plurality of optical transmitters coupled to an output of theclock and bit recovery circuit, wherein the optical transmitters areeach coupleable to a digital remote unit or a digital expansion unit.32. The system of claim 31, further comprising a wireless interfacedevice coupled to the digital host unit.
 33. The system of claim 32,wherein the wireless interface device comprises a base station thatcouples directly to the digital host unit via coaxial cables.
 34. Thesystem of claim 32, wherein the wireless interface device comprises abase station that wirelessly connects to the digital host unit via abi-directional amplifier that is coupled to an antenna.
 35. The systemof claim 31, wherein at least one digital remote unit is to coupled thedigital host unit.
 36. The system of claim 31, wherein at least onedigital remote unit is coupled to one of the at least two digitalexpansion units.
 37. The system of claim 31, wherein the at least onedigital expansion units are each coupled to the digital host unit via amultimode fiber pair.
 38. The system of claim 31, wherein the at leastone digital expansion unit is coupled to the digital host unit via asingle mode fiber.
 39. The system of claim 31, wherein the digital hostunit includes a radio frequency to digital converter that converts amain radio frequency signal to a digitized radio frequency signal. 40.The system of claim 39, wherein the digital host unit further includes amultiplexer which splits the digitized radio frequency signal into atleast two digital signals for optical transmission to the at least twodigital remote units.
 41. The system of claim 31, wherein the digitalhost unit includes local oscillators coupled to a reference oscillatorfor synchronization of the radio frequency signal in the forwarddirection and in the reverse direction.
 42. A digital radio frequencytransport system, comprising: a digital host unit; at least one digitalexpansion unit coupled to the digital host unit; and at least twodigital remote units, each coupled to one of the digital host unit andthe digital expansion unit, wherein the digital host unit includesshared circuitry that performs bi-directional simultaneous digital radiofrequency distribution between the digital host unit and the at leasttwo digital remote units; wherein the at least one digital expansionunit comprises: a plurality of optical receivers; a plurality of clockand bit recovery circuits each coupled to an output of one of theplurality of optical receivers; a plurality of demultiplexers eachcoupled to an output of one of the clock and bit recovery circuits; asummer coupled to an output of each of the plurality of demultiplexers;a multiplexer coupled to an output of the field programmable gate array;and an optical transmitter coupled to an output of the multiplexer. 43.The system of claim 42, wherein the summer includes a field programmablegate array that sums together digital radio frequency signals from theplurality of demultiplexers.
 44. The system of claim 42, wherein thefield programmable gate array includes an overflow algorithm circuit,which allows the summed digital radio frequency signals to saturate andkeeps the summed signal within a defined number of bits.
 45. The systemof claim 44, wherein the overflow algorithm circuit comprises a limiter.46. A digital radio frequency transport system, comprising: a digitalhost unit; at least one digital expansion unit coupled to the digitalhost unit; and at least two digital remote units, each coupled to one ofthe digital host unit and the digital expansion unit, wherein thedigital host unit includes shared circuitry that performs bi-directionalsimultaneous digital radio frequency distribution between the digitalhost unit and the at least two digital remote units; wherein the digitalhost unit includes: at least two optical receivers each coupled to oneof the at least two digital expansion units; at least two clock and bitrecovery circuits each coupled to an output of one of the at least twooptical receivers; and at least two demultiplexers each coupled to anoutput of one of the at least two clock and bit recovery circuits. 47.The system of claim 46, wherein the digital host unit further includes asummer coupled to an output of each of the at least two demultiplexers,wherein the summer receives a digital radio frequency signal from eachof the at least two demultiplexers.
 48. The system of claim 47, whereinthe summer includes a field programmable gate array that sums togetherthe digital radio frequency signals from each of the at least twodemultiplexers.
 49. The system of claim 40, wherein the fieldprogrammable gate array includes an alarm/control interface circuit. 50.The system of claim 40, wherein the field programmable gate arrayincludes an overflow algorithm circuit, which allows the summed digitalradio frequency signals to saturate and keeps the summed signal within adefined number of bits.
 51. The system of claim 50, wherein the overflowalgorithm circuit comprises a limiter.
 52. The system of claim 50,wherein the digital host unit further includes a digital to radiofrequency converter coupled to an output of the field programmable gatearray, wherein the radio frequency converter receives the summed digitalradio frequency signal and converts it to an analog radio frequencysignal.
 53. The system of claim 52, wherein the digital to radiofrequency converter comprises a digital to analog converter.
 54. Amethod of performing multipoint-to-point digital radio frequencytransport, the method comprising: receiving analog radio frequencysignals at multiple digital remote units; converting the analog radiofrequency signals to digital radio frequency signals at each of thedigital remote units; optically transmitting the digital radio frequencysignals from each of the digital remote units to a digital host unit;receiving the multiple digital radio frequency signals at the digitalhost unit; summing the multiple digital radio frequency signalstogether; and converting the digital radio frequency signals back toanalog radio frequency signals and transmitting the signals to awireless interface device for further transmission to a switchedtelephone network; wherein the digital host unit includes: an opticalreceiver; a clock and bit recovery circuit coupled to the opticalreceiver; and a demultiplexer coupled to the clock and bit recoverycircuit.
 55. The method of claim 54, wherein converting the analog radiofrequency signals to digital radio frequency signals comprisesamplifying the analog radio frequency signals.
 56. The method of claim54, wherein converting the analog radio frequency signals to digitalradio frequency signals comprises synchronizing a reverse path localoscillator to a master clock so as to reduce end-to-end frequencytranslation.
 57. A method of performing point-to-multipoint digitalradio frequency transport, the method comprising: receiving radiofrequency signals at a digital host unit; converting the radio frequencysignals to a digitized radio frequency spectrum; optically transmittingthe digitized radio frequency spectrum to a plurality of digital remoteunits; receiving the digitized radio frequency spectrum at the pluralityof digital remote units; converting the digitized radio frequencyspectrum to analog radio frequency signals; and transmitting the analogradio frequency signals via a main radio frequency antenna at each ofthe plurality of digital remote units; wherein the digital host unitincludes: an optical receiver; a clock and bit recovery circuit coupledto the optical receiver; and a demultiplexer coupled to the clock andbit recovery circuit.
 58. The method of claim 57, wherein converting theradio frequency signals to a digitized radio frequency spectrumcomprises amplifying the radio frequency signals.
 59. The method ofclaim 57, wherein converting the radio frequency signals to a digitizedradio frequency spectrum digital radio frequency signals comprisessynchronizing a forward path local oscillator to a reference oscillatorso as to reduce end-to-end frequency translation.
 60. The method ofclaim 57, wherein converting the digitized radio frequency spectrum toanalog radio frequency signals comprises amplifying the analog radiofrequency signals.
 61. The method of claim 57, wherein converting thedigitized radio frequency spectrum to analog radio frequency signalscomprises synchronizing a forward path local oscillator to a referenceoscillator so as to reduce end-to-end frequency translation.
 62. Amethod of performing point-to-multipoint digital radio frequencytransport, the method comprising: receiving radio frequency signals at adigital host unit; converting the radio frequency signals to a digitizedradio frequency spectrum; optically transmitting a first digitized radiofrequency spectrum to a first plurality of digital remote units and atleast one digital expansion unit; receiving the digitized radiofrequency spectrum at the first plurality of digital remote units;converting the digitized radio frequency spectrum to analog radiofrequency signals at each of the first plurality of digital remoteunits; receiving the digital radio frequency signals at the at least onedigital expansion unit; optically transmitting a second digitized radiofrequency spectrum to a second plurality of digital remote units;receiving the second digitized radio frequency spectrum at the secondplurality of digital remote units; converting the second digitized radiofrequency spectrum to analog radio frequency signals at each of thesecond plurality of digital remote units; and transmitting the analogradio frequency signals via a main radio frequency antenna at each ofthe first and second plurality of digital remote units; wherein thedigital host unit includes: an optical receiver; a clock and bitrecovery circuit coupled to the optical receiver; and a demultiplexercoupled to the clock and bit recovery circuit.
 63. A digital radiofrequency transport system, comprising: a digital host unit; and atleast two digital remote units coupled to the digital host unit, whereinthe at least two digital remote units each include: a main radiofrequency antenna; a duplexer coupled to the main radio frequencyantenna which receives radio frequency signals in the reverse path andtransmits radio frequency signals in the forward path; a radio frequencyto digital converter coupled to the duplexer in the reverse path; adigital to radio frequency converter coupled to the duplexer in theforward path; a multiplexer chip set coupled to the radio frequency todigital converter in the reverse path and the digital to radio frequencyconverter in the forward path; an optical transmitter coupled to anoutput of the multiplexer chip set; and an optical receiver coupled toan input of the multiplexer chip set; wherein the digital host unitincludes shared circuitry that performs bi-directional simultaneousdigital radio frequency distribution between the digital host unit andthe at least two digital remote units; wherein the digital host unitincludes: at least two optical receivers each coupled to one of the atleast two digital remote units; at least two clock and bit recoverycircuits each coupled to an output of one of the at least two opticalreceivers, and at least two demultiplexers each coupled to an output ofone of the at least two clock and bit recovery circuits.
 64. The systemof claim 63, wherein the digital host unit includes an alarm/controlinterface circuit.
 65. The system of claim 63, wherein the digital hostunit further includes a field programmable gate array coupled to anoutput of each of the at least two demultiplexers, wherein the fieldprogrammable gate array receives a digital radio frequency signal fromeach of the at least two demultiplexers.
 66. The system of claim 65,wherein the field programmable gate array includes a summer that sumstogether the digital radio frequency signals from each of the at leasttwo demultiplexers.
 67. The system of claim 66, wherein the fieldprogrammable gate array further includes an overflow algorithm circuit,which allows the summed digital radio frequency signals to saturate andkeeps the summed signal within a defined number of bits.
 68. The systemof claim 67, wherein the overflow algorithm circuit comprises a limiter.69. The system of claim 66, wherein the digital host unit furtherincludes a digital to radio frequency converter coupled to an output ofthe field programmable gate array, wherein the radio frequency converterreceives the summed digital radio frequency signal and converts it to ananalog radio frequency signal.
 70. The system of claim 69, wherein thedigital to radio frequency converter comprises a digital to analogconverter.
 71. A digital radio frequency transport system, comprising: adigital host unit, wherein the digital host unit includes: a radiofrequency to digital converter; a multiplexer coupled to an output ofthe radio frequency to digital converter; a plurality of opticaltransmitters coupled to an output of the multiplexer a first localoscillator coupled to an input of the radio frequency to digitalconverter; a reference oscillator coupled to an input of the first localoscillator; a second local oscillator coupled to an output of thereference oscillator; a digital to radio frequency converter coupled toan output of the second local oscillator; a channel summer coupled to aninput of the digital to radio frequency converter; a plurality ofdemultiplexers coupled to the channel summer; a plurality of clock andbit recovery circuits, wherein each of the plurality of clock and bitrecovery circuits are coupled to one of each of the plurality ofdemultiplexers; and a plurality of optical receivers, wherein each ofthe plurality of optical receivers are coupled to one of each of theplurality of clock and bit recovery circuits; at least two digitalremote units each coupled to one of the plurality of optical receivers,wherein the digital host unit includes shared circuitry that performsbi-directional simultaneous digital radio frequency distribution betweenthe digital host unit and the at least two digital remote units.
 72. Thesystem of claim 71, wherein the at least two digital remote units eachcomprise: an optical receiver; a clock and bit recovery circuit coupledto an output of the optical receiver; a demultiplexer coupled to anoutput of the clock and bit recovery circuit; a digital to radiofrequency converter coupled to an output of the demultiplexer; aduplexer coupled to an output of the digital to radio frequencyconverter; and a main radio frequency antenna coupled to an output ofthe duplexer.
 73. The system of claim 72, wherein the at least twodigital remote units each further comprise: a duplexer coupled to themain radio frequency antenna; a radio frequency to digital convertercoupled to an output of the duplexer; a multiplexer coupled to an outputof the radio frequency to digital converter; and an optical transmittercoupled to an output of the multiplexer.
 74. The system of claim 73,wherein the digital to radio frequency converter includes a digital toanalog converter.
 75. The system of claim 73, wherein the radiofrequency converter includes an analog to digital converter.
 76. Thesystem of claim 73, wherein the at least two digital remote units eachfurther comprise a programmable logic circuit coupled to an output ofthe radio frequency to digital converter and a reference oscillatorcoupled between the programmable logic circuit and the clock and bitrecovery circuit, wherein the programmable logic circuit monitors thesystem for error conditions and reports error modes.
 77. The system ofclaim 76, wherein the at least two digital remote units each furthercomprise a synthesizer circuit coupled to an output of the programmablelogic device and a first and a second local oscillator coupled to afirst and a second output, respectively, of the synthesizer.
 78. Thesystem of claim 71, wherein the at least two digital remote units eachcomprise: a main radio frequency antenna; a duplexer coupled to the mainradio frequency antenna; a radio frequency to digital converter coupledto an output of the duplexer; a multiplexer coupled to an output of theradio frequency to digital converter; and an optical transmitter coupledto an output of the multiplexer.
 79. A digital radio frequency transportsystem, comprising: a digital host unit, wherein the digital host unitincludes a channel summer; at least one digital expansion unit coupledto the digital host unit; and at least two digital remote units coupledto the digital host unit, wherein the digital host unit includes sharedcircuitry that performs bi-directional simultaneous digital radiofrequency distribution between the digital host unit and the at leasttwo digital remote units; wherein the digital host unit includes: anoptical receiver; a clock and bit recovery circuit coupled to theoptical receiver; and a demultiplexer coupled to the clock and bitrecovery circuit.